The European Commission has opened an investigation into whether Ryanair had benefited from illegal state aid at Germany's Frankfurt-Hahn airport.
"We will investigate whether regional and local authorities in Germany, against the rules, gave an unfair advantage to Ryanair over its competitors," EU Competition Commissioner Margrethe Vestager said in a statement.
Under the deal, Ryanair received between 2009 and 2017 training aid and funding for a crew and pilot school and for a maintenance hall at the airport, the Commission said.
Ryanair said it was confident that the Frankfurt-Hahn agreement fully complied with EU state aid rules.
"The EU has previously investigated this matter in 2014 and ruled that there was no state aid involved in the Ryanair-Frankfurt-Hahn agreement," the airline said in a statement.
The previous investigation related to arrangements before 2009.
The EU executive opened a similar investigation in July over a deal between Ryanair and the French airport of Montpellier.
The Commission said it will also investigate possible state aid to airport operator Flughafengesellschaft Frankfurt-Hahn), which between 2009 and 2017 was controlled by the German state of Rhineland-Palatinate.